1. Field of the Invention
This invention relates generally to spiral antenna systems and more specifically to an array of spiral antennas compactly mounted with improved directional pattern characteristics.
2. Prior Art
Spiral antennas existing in the prior art consist essentially of a spiral of conductor formed or otherwise attached to a flat surface. The spirals themselves come in many patterns and shapes from the Archimedian and equiangular spirals to the sawtoothed vanishing square.
Regardless of the pattern of the spiral, a spiral antenna with no reflecting surface placed in its radiation pattern will radiate a broad beam in two directions perpendicular to its face. Likewise, reception of radiation energy would be from both directions perpendicular to its face. Consequently, a spiral antenna is often provided with a cavity on one side to increase directional sensitivity. An example of a spiral antenna with a cavity is U.S. Pat. No. 3,192,531.
An array of spiral antennas each with its reflector and resonance cavity has been used for direction finding purposes or for the purpose of achieving desired directional patterns. The antennas are spaced about points of the compass for discrimination of angle of arrival by the relative signal level received on each spiral.
A problem inadequately addressed in the prior art is how to make such an array of spiral antennas compact. An array of discrete spiral antennas does not, in conventional systems, put to effective use the space it occupies. Prior approaches also resulted in differences between directional patterns of individual antennas causing direction finding errors.
It is an object of this invention to make a compact spiral antenna array with more uniform directional pattern performance.